1. Field of the Invention
The invention relates in general to transport refrigeration systems, such as refrigeration systems for trucks, trailers and containers, and more specifically to methods and apparatus for monitoring and protecting transport refrigeration systems.
2. Description of the Prior Art
Transport refrigeration systems condition a load space to a selected set point temperature, with the set point temperature being selected according to the nature of the load in the cargo space. For example, the load may be a frozen load, in which case the selected set point temperature will be below 15 degrees F., or it may be a perishable load having a selected set point temperature which is normally above freezing.
When the selected set point temperature is above a predetermined temperature, called the heat lock-out temperature, which may be 15 degrees F., for example, the transport refrigeration system controls the temperature of the load space to the selected set point by heating and cooling modes or cycles. When the selected set point is at or below the heat lock-out temperature, the heating mode or cycle is locked out.
U.S. Pat. No. 4,395,886, which is assigned to the same assignee as the present application, discloses a refrigerant charge monitor which detects the temperature differential of the refrigerant fluid on both sides of an expansion valve. While this arrangement is effective for refrigerant charge, it would be desirable to be able to monitor additional critical parameters of the refrigeration system and the load space, all with a single monitoring system.
For example, it would be desirable to be able to monitor transport refrigeration systems while they are operative to insure that when the control of a transport refrigeration system selects a heating or a cooling mode, that the system actually responds and provides heating or cooling, as required, in the cargo space. It would also be desirable to detect a malfunction of a transport refrigeration system which, even though the commanded and actual conditioning modes are consistent, will detect diminished capacity or performance, i.e., loss of efficiency. It would be desirable to detect insufficient refrigerant charge, notwithstanding that there is no malfunction in the control or the individual components of a transport refrigeration system. It would be desirable to detect and to protect the conditioned load against an extended defrost cycle. Finally it would be desirable to detect and protect the refrigerant compressor against situations which may cause damage to the compressor, notwithstanding that the temperature of the conditioned load and the selected set point are consistent.